This invention relates to the construction of an iris employed in an electromagnetic wave structure and, more particularly, to the construction of an iris with thermal stress relief grooves which allow for differential expansion of a metallic plate from which the iris is constructed relative to a metallic housing of the electromagnetic wave structure.
Irises are commonly employed in the coupling of electromagnetic waves between chambers in a structure which supports traveling and/or standing waves. One example of an electromagnetic wave structure of considerable interest herein is a resonator used in the construction of a microwave filter. For example, the filter may comprise two cylindrical chambers enclosed within the cylindrical metallic wall of a housing of the filter. Each chamber is provided with an end metallic wall, the two chambers being coupled by one or more coupling apertures in an iris disposed at a central location of the housing between the two end walls. In the event that both chambers are to resonate at the same frequency, the iris is positioned equally distant between the two end walls. In the event that the chambers are to resonate at slightly different frequencies, then the location of the iris may be offset slightly from the central location between the two end walls.
Particularly in the case of resonators employed in microwave filters, it has been the practice to construct the metallic walls of the filter of a metal which has a low coefficient of thermal expansion so as to minimize changes in the physical dimensions of the filter in the presence of changing temperature. For example, in the event that the microwave filter is used in the transmission of intense electromagnetic power, a significant amount of heating occurs within the walls of the filter. The heating produces expansion of the housing and other elements of the filter with a resultant shift in the resonance frequency of the various resonators or chambers within the filter. The electrically conductive metal, a 36% nickel steel alloy commonly sold under the name Invar, Invar alloy is frequently employed because of its very low coefficient of thermal expansion.
However, a problem arises in that a metal, such as aluminum, is preferable for the construction of the iris because such metal is of lighter weight, has better heat flow properties, and is easier to machine than a metal such as Invar. Therefore, it would be preferable to construct the iris of a plate of aluminum. However, due to the much larger coefficient of thermal expansion of aluminum, as compared to the relatively low thermal coefficient of expansion of Invar alloy, the aluminum expands much more than does the Invar alloy in the presence of heating of the filter, or other microwave structure in which the iris may be employed. As a result, the iris buckles, resulting in a distortion of the iris, and also presents an intrusion of a central portion of the iris into one of the chambers. This has the effect of a reduction in a longitudinal dimension of the chamber with an increase in the longitudinal dimension of the other chamber. As a result of the dimensional changes of the two chambers, the shortened chamber is detuned to a higher frequency and the lengthened chamber is detuned to a lower frequency. Also, distortions in the surface of the iris may result in an altered bandpass characteristic of each of the chambers. Thus, operation of the filter may be degraded significantly.